
The present invention relates to a process for producing hollow microcoils, or hollow microfibers that can be used as a material for producing a three-dimensional reinforcing composite material, electromagnetic wave absorbing material, electrode material, catalyst, catalyst carrier, adsorbing agent, micromechanical element, microswitching element, microsensor, microfilter and the like.
Fibers and whiskers (slender single crystals like whiskers of a cat) of carbon fibers, metal carbides or nitrides have heretofore been produced relying upon the metallic catalytically activated CVD method, the fibers and whiskers assuming the state of straight fibers which are completely solid no to the cores of the fibers. Hollow fibers in the form of micropipes have not at all been reported. Generally, the fibers and whiskers grow in vapor phase relying upon the VLS (gas-liquid-solid) mechanism With this mechanism, however, the fibers or whiskers are not theoretically allowed to grow into the form of pipes. As for producing the fibers in the form of microcoils, the present inventors have previously proposed a process for producing microcoiled carbon fibers by pyrolyzing acetylene in the presence of a metal catalyst and a trace amount of sulfur or phosphorus impurities (see Japanese Unexamined Patent Publication (Kokai) No. 222228/1992). The inventors have also proposed a process for producing a variety of microcoiled meal nitride fibers by subjecting the coiled carbon fibers to the vapor-phase metallization and to the nitrogenation simultaneously at a high temperature (still pending).
The microcoiled carbon fibers can be produced maintaining good reproducibility and at a high yield by pyrolyzing a hydrocarbon gas in the presence of a metal catalyst and a trace amount of sulfur or phosphorous impurities. The shape of the coils is generally irregular. Even the regularly wound coils are not intimately adhered to each other but include gaps. The Inventors therefore have closely studied the conditions for synthesizing the coiled carbon fibers and the after-treatment therefor, and discovered the fact that when the synthesizing conditions are highly strictly controlled, the carbon fibers are wound very regularly, completely filling up the gaps among the coils, making it possible to obtain hollow microcarbon fibers with hollow coil cores. The inventors have further studied a process for producing various hollow ceramic microfibers using the above-mentioned hollow microcarbon fibers as a starting material and have completed the present invention. It is also allowable to use hollow metal carbide microfibers, metal nitride microfibers and metal carbonitride fibers as a starting material. The object of the present invention is to provide a novel process for producing hollow microfibers of various intermetallic compounds and ceramic compounds.
The present invention relates to hollow carbon microcoils of which the fiber cores are hollow and the coils are intimately adhered to each other, to hollow ceramic microcoils or hollow ceramic microfibers obtained by subjecting the hollow carbon microcoils to the vapor-phase metallization silification, boration, carbonization, nitrogenation and/or oxidation in a mixed gas containing a metal-, silicon-, boron-, carbon-, nitrogen- or oxygen-containing gas at a high temperature, and to a process for producing the same. The invention further relates to hollow ceramic microcoils or hollow ceramic microfibers obtained by subjecting the hollow metal carbide microcoils or the metal nitride microcoils formed by metallizing the hollow carbon microcoils to the vapor-phase metallization, silicification, boration, carbonization, nitrogenation and/or oxidation in a mixed gas containing a metal-, silicon-, boron-, carbon-, nitrogen- or oxygen-containing gas at a high temperature, and to a process for producing the same.
More specifically, the invention provides hollow carbon microcoils which are completely and intimately adhered to each other as a result of highly strictly controlling the conditions of synthesis at the time of synthesizing the carbon microcoils, and provides hollow ceramic microcoils which are obtained by subjecting the hollow carbon microcoils as a starting material to the vapor-phase diffusion reaction in a mixed gas containing a metal-, silicon-, boron-, carbon-, nitrogen- and/or oxygen-containing gases at a high temperature, the hollow ceramic microcoils being composed of an intermetallic compound, a metal silicate, a metal boride, a metal carbide, a metal nitride, a metal oxide or a composite compound thereof in which part or whole of carbon atoms are physically or chemically treated with the above-mentioned components.
The invention further provides hollow ceramic microfibers in which the neighboring coil-forming portions of the microcoils are partly or wholly melt-adhered together, so that the microcoils partly or wholly assume the fibrous form as a result of the above-mentioned vapor-phase diffusion reaction at a high temperature.
The invention further provides hollow ceramic microcoils or hollow ceramic microfibers which are obtained by subjecting the known hollow carbon microcoils, hollow carbon fibers or hollow carbon microfibers of which the coils are not intimately adhered to each other as a starting material to the vapor-phase treatment in one stage or in two or more stages in a mixed gas containing a metal-, silicon-, boron-, carbon-, nitrogen- and/or oxygen-containing gas at a high temperature, the hollow ceramic microcoils or the hollow ceramic microfibers being composed of an intermetallic compound, a metal silicate, a metal carbide, a metal nitride, a metal oxide or a composite compound thereof in which part or whole of carbon atoms are physically or chemically treated with the above-mentioned components, and a process for producing the same.